The broad aim of this study is to increase our understanding of the local flow processes which take place in the blood microcirculatory system (and its environs), and to quantitatively relate the behavior of such flows to the macroscopic rheological properties of blood. The research is seen as a progression of research starting with quantitative experimental studies of blood flow through single, long, artificial tubes whose inside diameters range from 4 microns up to several hundred microns (steady, oscillatory and pulsatile flows). The final level of research will involve quantitative, experimental in-vivo measurements on microcirculatory (and larger) systems. Intermediate studies involve in-vitro experimentation on blood flow through bifurcations, small networks of vessels, and large networks of vessels, using real-geometry and real-size replicas for vascular systems. At this time, extensive data on steady blood flow through single tubes have been obtained; the variables measured include tube dimensions; blood rheologcal properties; hematocrits in the feed reservoir, tube and outflowing blood; blood and red cell velocities; and pressure drops. Similar data are being obtained for oscillatory flow in single tubes, and for steady flow through larger bifurcations. It is proposed that the in-vitro studies, primarily using plastic replicas of real microcirculatory (and larger) systems be completed in the next 7-year period. The red cell deformability and aggregability will be altered, and the effects of these changes on the cell suspension's rheological properties and on local blood flow characteristics in the flow geometries mentioned above will be assessed.